The present invention relates to an optical scanning method and apparatus for directing and moving a spot or pattern of light, typically from a laser, across a line or an area on some given surface, for example, to access an area on a surface that moves linearly or angularly. The invention is particularly applicable to provide a rapid random-access optical scanner and method for optical disk data storage, and is therefore described below with respect to this application.
Optical disk data storage involves a number of somewhat conflicting requirements, including:
1. Fast Random-Access: In such data storage, the scanned line is not accessed in an orderly way, e.g., sequentially or predefined non-sequentially, but rather in a random manner known only during the actual operation of the system; random-access requires that the time to reach a requested point must be as short as possible (preferably around a millisecond or less).
2. High Resolution: That is, the size of the light spot formed by the scanning system (or, for some applications, the finest details in the pattern projected by the system) must be as small as possible, typically of the order-of-magnitude of the wavelength of the light; the precision of the scanning (the precision in the location of the spot) must be comparable with the spot size.
3. Large Space-Bandwith-Product: That is, the total number of points accessible by the scanning system should be large; this number roughly equals the length of the scan line divided by the spot size (a typical value should be at least in the thousands).
In addition, for some key applications, such as R/W (read/erase/write) and WORM (write-once read-many) optical computer disk systems, it is necessary to have:
4. High Light Power Concentration: That is, it must be possible to have enough energy delivered to a spot in a short time to affect a "write" operation.
For such R/W and WORM applications, it is desirable to focus most of the available laser beam power into a single spot for the "write" operation. Techniques for accomplishing this by illuminating many spots simultaneously (parallel access). or by optically encoding the data over an area on the disc (such as holographic storage). are described in a number of prior publications, including: Ph. Marachand. A. V. Krishnamoorthy, K. S. Urquhardt, P. Ambs, S. Esener and S. H. Lee, "Motionless Head for Parallel Readout Optical Disk," SPIE/IST Symp. on Electronic Imaging Science and Technology, February 1992, San Jose. Calif., published in Proc. SPIE, vol. 1662 (SPIE, Bellingham Wash., 1992); V. A. Ivanov, B. S. Kiselyov, A. L. Mikaelian and D. E. Okonov, "Optoelectronic Neuroprocessor Based on Holographic Disk Memory," Optical Memory and Neural Networks, vol. 1, pp. 55-62 (1992).
The standard solution is to put the entire laser/optics assembly (or most of it) at the end of a movable arm, just as is done today with magnetic heads for magnetic computer disks, and earlier with mechanical stylus pick-ups in phonograph players. However, laser/optics assemblies are heavier than magnetic disk heads. Therefore they cannot be accelerated and moved as quickly since the inertia of an object to rotation is proportional to the square of the distance of its mass from the center of rotation. As a result, optical disk systems are usually slower than magnetic disk ones.
Because light beams can be focussed and steered at a distance, it would be preferable to use a movable mirror (or some other beam steering device) away from the scanned surface and to keep the laser and most of the optics stationary. However, the combination of requirements 2 and 3 above, and the physical laws of optical diffraction, would dictate a rather large and heavy mirror. Such a mirror cannot be accelerated quickly and would therefore be incompatible with the fast random-access requirement 1.
There is thus an urgent need for a high resolution optical scanning method and apparatus which provide fast random-access, comparable to or better than that of a non-optical magnetic disk scanning system.